The Fischer-Tropsch (FT) process (or Fischer-Tropsch synthesis) is a catalyzed chemical reaction in which synthesis gas (a mixture of carbon monoxide and hydrogen) is converted into liquid hydrocarbons. A variety of catalysts have been used for the FT process, but the most common active metals are based on cobalt or iron. In addition, to the active metals, noble metal promoters such as ruthenium or rhenium are added to enhance activity/selectivity of the active ingredient. Typically, the active metal and the promoters are supported on alumina, silica or titania.
The FT process can be carried out either in a fixed bed reactor or a slurry bed reactor. In case of a slurry bed process, the catalyst particles are suspended in oil with gaseous reactants being bubbled into the reactor. For either process to be economically viable, the catalyst must exhibit good performance for a long period of time without significant loss in catalytic activity. Typically, catalyst deactivates because of one or more of the following issues: (a) poisoning of the active catalytic metal (e.g., cobalt), (b) loss of catalytic metal surface area (e.g., via sintering), (c) loss of active metal species due to reaction with support, and (d) attrition.
The attrition of the catalyst, i.e., issue (d) above, is primarily dependent on the strength of the support for the catalytically active metal. Using slurry bed catalyst as an example regarding the attrition issue, the particles in these catalysts are subjected to a number of collisions either with other particles or with the reactor walls. This causes the catalyst particles to “attrit” or break into smaller particles. Smaller particles are not retained in the reactor, and as a result the activity declines absent continuous addition of fresh catalyst. In order to enhance performance of the catalyst and to improve the catalyst life, a support must therefore exhibit high attrition resistance.
High surface area alumina is commonly used as a catalyst support for FT. Supports having high surface area provide the necessary support surface for dispersing catalytic sites throughout the catalyst. High surface area aluminas are conventionally prepared by calcining an aluminum hydroxide composition such as boehmite. Calcined, high surface area alumina per se, however, does not exhibit good attrition resistance. Indeed, it is also largely believed that aluminas after calcination cannot be easily bound into hard particles. Hence there is a tendency to use boehmitic aluminas as support precursors, which are slurried in water and “peptized” in the presence of an acid such as nitric or hydrochloric acid, followed by drying and calcinations to give attrition resistant particles. This alternative presents its own problem because these peptized boehmitic aluminas slurries gel at high solids content and need to diluted before drying and calcination. Processing the alumina at high solids content is desirable not only to get high production rates, but also to yield a strong particle of desired particle size upon spray drying.
In addition, and as referenced with respect to issue (c) above, high surface area alumina supports react with active metal precursor of cobalt to form Co-aluminate spinel upon calcination. This transforms the active Co metal to “inactive” spinel Co-aluminate and thus decreases the catalyst activity.
In order to prevent Co-aluminate spinel formation, divalent metals like Ni, Zn, and Mg can be added to an alumina support to form the spinel phase “a priori” and thus prevent the formation of inactive Co-aluminate. The divalent metal aluminate spinels are formed upon high temperature calcinations above 650° C. Such spinel materials do not exhibit high strength, however, and can easily break into smaller particles. In other words, such spinel phase-based particles generally do not have sufficient attrition resistance.
It has been shown that if the spinel compositions are calcined at very high temperatures, in excess of 1100° C., the attrition resistance improves significantly (see WO 2005/072866 A1 or US 2007/0161714). In addition to requiring high calcination temperatures, it is also apparent that high levels of divalent metals are needed to attain the good attrition resistance. Typically, the divalent compound is in excess of 10 wt % (as metal) in loading.
It has also been shown that, as a result of high temperature calcinations, the support pore diameter shifts to higher pore modes. Catalysts made from these high temperature calcined spinel supports therefore have high selectivity to higher hydrocarbons in addition to the aforementioned attrition resistance. The practical use of these supports, however, is limited due to expensive processing steps, and large amounts of expensive divalent metal compounds added as dopants. Furthermore, large amount of divalent dopant compounds poses the risk of leaching out of the spinel structure and adversely affecting the catalyst activity.
The object of this invention therefore is to provide catalysts with improved attrition resistant metal aluminate spinel supports made at relatively low temperatures and preferably using relatively low levels of divalent metals.